The present invention relates to a unit for transmitting motion to a hosiery suction tube in a circular knitting machine for manufacturing hosiery, i.e., in small-diameter circular knitting machines.
Conventional circular knitting machines for manufacturing hosiery include a needle cylinder and a sinker ring that are coaxial, are made to rotate continuously, and move, within respective slots, needles and sinkers that are actuated in a coordinated manner so as to gradually form a tubular knitted fabric that falls along an internal tube that is coaxial to the cylinder and is known as the hosiery passage tube.
Depending on the type of thread being processed, or due to various kinds of textile requirement (especially in the field of women's hosiery), sometimes the hosiery passage tube must be fixed or must be rotated together with the needle cylinder.
The hosiery item passes through the hosiery passage tube by suction from below, but in cases in which the hosiery passage tube remains fixed while the cylinder rotates, the hosiery tends to become twisted; in this case the hosiery item, which rotates together with the needles in the top part, in fact rubs against the internal surface of the tube, which is kept fixed, and despite the presence of the air stream, beyond a certain length the friction forces against the tube walls, which tend to cause twisting, prevail over the forces of the descending air stream, which would otherwise tend to keep the hosiery item stretched.
In order to obviate this drawback, modern knitting machines have been equipped with a device, known as an "anti-twist" device, that is constituted by a perforated tubular element that is arranged below the hosiery passage tube and coaxially thereto; the suction air passes through said perforated tube, which is rotated at the same speed as the needle cylinder, and the suction of the air through its surface tends to draw the hosiery item against its walls, so as to rigidly couple the hosiery item to a surface that rotates together with the needle cylinder and thus prevents it from becoming twisted.
Transmissions based on gears or belts and driven by the cylinder support or by the motor are usually used to rotate the perforated tube of conventional "anti-twist" devices: such actuation systems have considerable drawbacks, due to noise, difficulty in lubrication, and bulk; even greater costs and drawbacks occur in the case of knitting machines driven by a hollow motor that is coaxial to the cylinder.
In addition to this, it is rather troublesome to change the hosiery passage tube from the rotating condition to the non-rotating condition, or vice versa, and selection can usually be performed only when the machine is not working.